1. Field of the Invention
This invention relates to a phototube which is operative even in a strong magnetic field and to a radiation detecting device using the phototube, specifically to those which are used in the field of high energy physics.
2. Related Background Art
Conventionally the phototube of this kind has the structure of FIG. 1. A photocathode 2 is formed on the inside surface opposed to the light incident surface 1a of a glass container 1. A beam of light from the light incident surface 1a is converted into photoelectrons by the photocathode 2. The converted photoelectrons are attracted to an anode 3 opposed to the photocathode 2 by an electric field E to be captured by the anode 3.
Some of the phototubes having the photoelectron multiplying function, i.e., including photoelectron multipliers, have the structure of FIG. 2. A photocathode 52 is formed on the inside surface opposed to the light incident surface 51a of a glass container 51. A beam of light from the light incident surface 51a is converted into photoelectrons by the photocathode 52. The converted photoelectrons are attracted to an anode 53 opposed to the photocathode 52 by an electric field E. A dynode 53 is made up with a plurality of electrodes 53a, b, . . . and emits in secondary electrons the photoelectrons it received. The emitted secondary electrons are finally captured by an anode 54.
But in the above-described conventional phototube 4, if a strong magnetic field B is absent in the direction normal to the electric field E and in the vertical direction in the drawing, in other words, the direction parallel with the light incident surface 1a, although it depends on a voltage applied between the photocathode 2 and the anode 3, there will occur the phenomenon that the photoelectrons emitted by the photocathode 2 cannot be captured by the anode 3. That is, because of the strong magnetic field B, the photoelectrons emitted by the photocathode 2 have a cycloidal motion, a circular motion returning to the photocathode 2 as indicated by the arrow. Consequently the photodetecting efficiency is sometimes lowered because of the strong magnetic field B normal to the electric field E.
This phenomenon is true also with the above-described conventional photoelectron multiplying tube 55. That is, if a strong magnetic field B is present in the direction parallel with the light incident surface 51a of the glass container 51, although it depends on a voltage applied between the photocathode 52 and the electrodes 53a, b, . . . of the dynode 53a, the photoelectrons emitted by the photocathode 52 cannot be captured by the dynode 53a. In other words, similarly with the above-described phototube, the photoelectrons emitted by the photocathode 52 have a cycloidal motion, a circular motion returning to the photocathode 52 as indicated by the arrow. This phenomenon takes place also at the part of the dynode 53 which multiplies the electrons. Consequently the photodetecting efficiency of the photoelectron multiplying tube 55 is sometimes lowered because of the strong magnetic field B normal to the electric field E.
FIG. 3 is a diagrammatic view of the cycloidal motion of photoelectrons in the above-described phototubes. The track of the photoelectrons is depicted by the dot line. For example, when a strength of the magnetic field B is 0.6 [T], an initial velocity of the emitted photoelectrons is 0 [eV], an applied voltage between the photocathode 2 and the anode 3 or between the photocathode 52 and the dynode 53a is 1000 [V], the photoelectrons are spaced by 0.177 [mm] at maximum (=y.sub.max) from the photocathode 2 or 52 because of the cycloidal motion. Accordingly under these set conditions, if the photocathode 2 and the anode 3 or between the photocathode 52 and the dynode 53a are spaced from each other by more than 0.177 [mm], the photoelectrons emitted by the photocathode 2 or 52 cannot arrive at the anode 3 or the dynode 53a. In the photoelectron multiplying tube 55, such influence of the strong magnetic field B range not only between the photocathode 52 and the dynode 53a, but also to the electron multiplication of the dynode 53a.
To solve this problem, the phototube 4 or the photoelectron multiplying tube 55 itself is so moved or turned in accordance with a direction of the magnetic field B known beforehand that the magnetic field B is normal to the light incident surface 1a or 51a. This is because when the electric filed E and the magnetic field B are parallel with each other, the photoelectrons do not have a cycloidal motion, and resultantly the photodetecting efficiency of the phototube 4 and the photoelectron multiplying tube 55 is not lowered.
But in the high energy particles (radiation) detecting devices using such phototube 4 or photoelectron multiplying tube 55, as described above it is generally difficult to optimumly change positions and directions of the phototube 4 or the photoelectron multiplying tube 55. That is, such radiation detecting device usually has a section of FIG. 4. A plurality of scintillators 5 of BaF.sub.2 for detecting radiation are arranged so as to enclose the detecting portion for a light beam to pass through, and the photocathode 4 or the photoelectron multiplying tube for photoelectrically converting detected radiation is fixed to the back of each of the scintillators. Accordingly the position of the photocathode 4 or the photoelectron multiplying tube 55 itself is restricted by its connection to the output terminal of each of the scintillators 5 and cannot be optionally changed. Consequently to position the photocathode 4 or the photoelectron multiplying tube 55 so that the magnetic field B is normal to the light incident surface 1a or 51a of the glass container 1 or 51, it is not necessary to machine the scintillators 5 in a rod shape but to machine them 5 so that the output terminals form a required angle. Generally, however, it is difficult to machine scintillators. Consequently it is actually impossible to agree a direction of the electric field E generated between the photocathode 2 and the anode 3 or between the photocathode 52 and the dynode 53a with a direction of the magnetic field B.